According to Emergency Situations and Disaster Management Department, the powerful explosion took place at 1010 IST in the second coach of the train going from Paveltskaya station in the centre of the town to Avtozavodskaya.

A fire followed the blast and smoke was bellowing out of Avtozavodskaya station as about 50 ambulance vans arrivedthere.

More than 500 passengers have been evacuated and a massive rescue operation is on, ITAR-TASS reported.

The train wagon was badly damaged in the explosion.

Police cordoned off the two metro stations and stopped all traffic between the stations.

Police spokesman Kiril Mazurim said the blast was the result of a suicide bomb attack. However, he said it was difficult at the moment to identify whether the bomber was a male or a female.

Forensic and explosive experts are investigating the the blast, he said.

About 700 passengers have been evacuated from the site, ITAR-TASS reported.